Low cost lead-free preplated leadframe having improved adhesion and solderability

ABSTRACT

A leadframe with a structure made of a base metal ( 105 ), wherein the structure has a plurality of surfaces. On each of these surfaces are metal layers in a stack adherent to the base metal. The stack comprises a nickel layer ( 201 ) in contact with the base metal, a palladium layer ( 202 ) in contact with the nickel layer, and an outermost tin layer ( 203 ) in contact with the palladium layer. In terms of preferred layer thicknesses, the nickel layer is between about 0.5 and 2.0 μm thick, the palladium layer between about 5 and 150 nm thick, and the tin layer less than about 5 nm thick, preferably about 3 nm. At this thinness, the tin has no capability of forming whiskers, but offers superb adhesion to polymeric encapsulation materials, improved characteristics for reliable stitch bonding as well as affinity to reflow metals (solders).

FIELD OF THE INVENTION

The present invention is related in general to the field ofsemiconductor devices and processes, and more specifically to thematerials and fabrication of leadframes for semiconductor devices.

DESCRIPTION OF THE RELATED ART

Leadframes for semiconductor devices provide a stable support pad forfirmly positioning the semiconductor chip, usually an integrated circuit(IC) chip within a package. Since the leadframe, including the pad, ismade of electrically conductive material, the pad may be biased, whenneeded, to any electrical potential required by the network involvingthe semiconductor device, especially the ground potential.

In addition, the leadframe offers a plurality of conductive segments tobring various electrical conductors into close proximity of the chip.The remaining gap between the inner end of the segments and the contactpads on the IC surface are bridged connectors, typically thin metallicwires individually bonded to the IC contact pads and the leadframesegments. Consequently, the surface of the inner segment ends has to besuitable for stitch-attaching the connectors.

Also, the ends of the lead segment remote from the IC chip (“outer”ends) need to be electrically and mechanically connected to externalcircuitry, for instance, to assemble printed circuit boards. In manyelectronic applications, this attachment is performed by soldering,conventionally with lead-tin (Pb/Sn) eutectic solder at a reflowtemperature in the 210 to 220° C. range. Consequently, the surface ofthe outer segment ends has to have affinity for reflow metals or alloys.

Finally, the leadframe provides the framework for encapsulating thesensitive chip and fragile connecting wires. Encapsulation using plasticmaterials, rather than metal cans or ceramic, has been the preferredmethod because of low cost. The transfer molding process for epoxy-basedthermoset compounds at 175° C. has been practiced for many years. Thetemperature of 175° C. for molding and mold curing (polymerization) iscompatible with the temperature of 210 to 220° C. for eutectic solderreflow.

Reliability tests in moist environments require that the moldingcompound have good adhesion to the leadframe and the device parts itencapsulates. Two major contributors to good adhesion are the chemicalaffinity of the molding compound to the metal of the leadframe and thesurface preparation of the leadframe.

The recent general trend to avoid Pb in the electronics industry and usePb-free solders, pushes the reflow temperature range into theneighborhood of about 260° C. This higher reflow temperature range makesit more difficult to maintain the mold compound adhesion to theleadframes. This is especially true for the very small leadframe surfaceavailable in QFN (Quad Flat No-lead) and SON (Small Outline No-lead)devices. For this temperature range, known leadframes do not offermetallization for good adhesion combined with low cost, easymanufacturability, and avoidance of whiskers.

It has been common practice to manufacture single piece leadframes fromthin (about 120 to 250 μm) sheets of metal. For reasons of easymanufacturing, the commonly selected starting metals are copper, copperalloys, and iron-nickel alloys (for instance the so-called “Alloy 42”).The desired shape of the leadframe is etched or stamped from theoriginal sheet. In this manner, an individual segment of the leadframetakes the form of a thin metallic strip with its particular geometricshape determined by the design. For most purposes, the length of atypical segment is considerably longer than its width.

SUMMARY OF THE INVENTION

A need has therefore arisen for a low cost, reliable leadframe combiningadhesion to molding compounds, bondability for connecting wires,solderability of the exposed leadframe segments, and no risk of tindendrite growth. There are technical advantages, when the leadframe andits method of fabrication is low cost and flexible enough to be appliedfor different semiconductor product families and a wide spectrum ofdesign and assembly variations, and achieves improvements toward thegoals of improved process yields and device reliability. There arefurther technical advantages, when these innovations are accomplishedusing the installed equipment base so that no investment in newmanufacturing machines is needed.

One embodiment of the invention is a leadframe with a structure made ofa base metal, wherein the structure has a plurality of surfaces. On eachof these surfaces are metal layers in a stack adherent to the basemetal. The stack comprises a nickel layer in contact with the basemetal, a palladium layer in contact with the nickel layer, and anoutermost tin layer in contact with the palladium layer.

In terms of preferred layer thicknesses, the nickel layer is betweenabout 0.5 and 2.0 μm thick, the palladium layer between about 5 and 150nm thick, and the tin layer less than about 5 nm thick, preferably about3 nm. At this thinness, the tin has no capability of forming whiskers,but offers superb adhesion to polymeric encapsulation materials,improved characteristics for reliable stitch bonding as well as affinityto reflow metals (solders). By replacing the conventional gold layer,the tin layer offers a low cost alternative.

Another embodiment of the invention is a semiconductor device, which hasa leadframe with a structure made of a base metal, wherein the structurecomprises a chip mount pad and a plurality of lead segments. The basemetal has consecutively an adherent nickel layer in contact with thebase metal, a palladium layer in contact with the nickel layer, and anoutermost tin layer in contact with the palladium layer. A semiconductorchip is attached to the mount pad, and conductive connections tie thechip and the lead segments. Polymeric encapsulation material covers thechip, the connections and portions of the lead segments.

Another embodiment of the invention is a method for fabricating aleadframe, wherein a base metal structure with a plurality of surfacesis provided. A stack of metal layers adherent to the base metal isplated on each of the structure surfaces. This plating step comprisesthe plating of a nickel layer, thickness between about 0.5 to 2.0 μm, incontact with the base metal; next, the plating of a palladium layer,thickness between about 5 and 150 nm, in contact with the nickel layer;and finally the plating of a tin layer, thickness less than about 5 nm,in contact with the palladium layer. All three plating steps can beperformed without masking or selective plating.

It belongs to the technical advantages of the invention that no toxic orwhiskering materials are used for the plating steps, down-bondingcapability is enhanced, and moisture-level quality is improved.Furthermore, the required plating processes are inexpensive and easy tomanufacture.

The technical advances represented by certain embodiments of theinvention will become apparent from the following description of thepreferred embodiments of the invention, when considered in conjunctionwith the accompanying drawings and the novel features set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of the base metal structure of aportion of a leadframe strip having formed leadframe structures.

FIG. 2 illustrates a schematic cross section of a leadframe stripportion with a base metal structure and a plurality of surfaces, whereinthe surfaces have been plated with a stack of adherent layers accordingto the invention.

FIG. 3 illustrates a device embodiment of the invention; the schematiccross section shows a portion of a leadframe strip, prepared accordingto an embodiment of the invention, and a plurality of semiconductorchips assembled and encapsulated on one leadframe surface.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic and simplified cross section of a leadframeportion, generally designated 100. The leadframe has a plurality ofsurfaces: the first surface 101, the second surface 102, and numerousside edge surfaces 110 a, 110 b . . . 110 n. While the surfaces 101 and102 originate from the sheet of starting material, the side edgesurfaces 110 a to 110 n have been created by the forming process of theleadframe structure. The leadframe portion depicted contains a pluralityof chip mount pads 103 and a plurality of lead segments 104. Theleadframe is made of a base metal 105.

As defined herein, the starting material of the leadframe is called the“base metal”, indicating the type of metal. Consequently, the term “basemetal” is not to be construed in an electrochemical sense (as inopposition to ‘noble metal’) or in a structural sense.

Base metal 105 is typically copper or a copper alloy. Other choicescomprise brass, aluminum, iron-nickel alloys (“Alloy 42”), and Kovar.

Base metal 105 originates with a metal sheet in the preferred thicknessrange from 100 to 300 μm; thinner sheets are possible. The ductility inthis thickness range provides the 5 to 15% elongation that facilitatesthe segment bending and forming operation needed for some of thefinished devices. The leadframe parts such as chip mount pads, leadsegments, connecting rails (not shown in FIG. 1, but hinted at by dashedlines) are stamped or etched from the starting metal sheet. As stated,these stamping or etching processes create numerous side edges 110 a,110 b . . . 110 n, of the leadframe parts.

FIG. 2 illustrates a schematic cross section of a leadframe according toan embodiment of the invention. The leadframe structure has a base metal105 with a plurality of surfaces, which were created by the process ofstamping or etching. A stack of metal layers adheres to each of thestructure surfaces. The stack comprises: A nickel layer 201 in contactwith base metal 105; the preferred thickness range of the nickel layeris between about 0.5 and 2.0 μm. Next, a palladium layer 202 is incontact with the nickel layer; the preferred thickness range of thepalladium layer is between about 5 and 150 nm. Finally, the outermostlayer 203 is a thin tin layer in contact with the palladium layer; thepreferred thickness of the tin layer is less than 5 nm, most preferablyabout 3 nm. The outermost thin tin enhances the adhesion to polymericencapsulants such as polyimides and epoxies significantly; for somemolding compounds, an adhesion improved by a factor of 15 compared tothe conventionally often used gold layer has been measured. In addition,the tin layer supports the affinity of the palladium to reflow metalssuch as solders (preferably tin and tin alloys such tin/silver/copper).Furthermore, the thinness of the tin layer allows reliable wire stitchbonding (for instance, gold wires) to the palladium, and has nopotential for growing whiskers because of its thinness.

Since all these metal layers cover the leadframe surfaces uniformly, thelayer deposition is preferably achieved by plating the whole leadframestrip (see below) and masking can be avoided. However, if some devicesrequire a tin layer with a plurality of windows in selected areas inorder to expose the underlying palladium for special metal bonding, thenthe tin layer has to be plated with a mask applied to leadframe surface101. An example of a resulting window is designated 240 in FIG. 2.

In another embodiment, some wire stitch bonding may need an additionalsilver layer over the tin layer in selected areas. In this case, thesilver has to be plated using a mask (see below). An example of aresulting selected silver spot is designated 230 in FIG. 2.

Another embodiment of the invention is a semiconductor device, asexemplified by the Quad Flat No-leads (QFN) or Small Outline No-leads(SON) device in FIG. 3. Actually, FIG. 3 shows a leadframe strip with aplurality of assembled and packaged devices before singulation. In theembodiment of the invention, the device has a leadframe with a basemetal 301 and a first surface 301 a and a second surface 301 b. Anexample for the base metal is copper. Furthermore, the leadframe isstructured into a chip mount pad 302 and a plurality of lead segments303. Each lead segment has a first end 303 a near chip mount pad 302,and a second end 303 b remote from mount pad 302.

The first leadframe surface 301 a, the second leadframe surface 301 b,and all side edges are covered by a stack of layers, which provides forthe leadframe reliable adhesion to polymeric materials as well asaffinity to reflow metals. The stack of layers comprises a nickel layer304 in contact with the base metal, a palladium layer 305 in contactwith the nickel layer, and an outermost tin layer 306 in contact withthe palladium layer.

A semiconductor chip 310, for example an integrated circuit chip, isattached by means of an adhesive layer 311 to each chip mount pad 302.Bonding wires 312 interconnect chip 310 with the first ends 303 a of thelead segments 303. The stitch bond 312 a is in welded to the outermosttin layer 306, where it typically breaks through the thin layer and isactually attached to the palladium layer 305. In some embodiments,selective silver areas 330 support the stitch attachments of wires 312.In other embodiments, it is advisable to leave windows 340 in theoutermost tin layer 306 for the stitch attachments to the palladiumlayer 305.

Polymeric encapsulation material 320, for example epoxy-based moldingcompound, covers chip 310, bonding wires 312 and first ends 303 a of thelead segments. The polymeric material 320 also fills the gaps betweenchip 310 and the first ends of the lead segments and thus covers theleadframe side edges. Consequently, polymeric material 320 also forms asurface 321 in the same plane as the outermost surface layer 306.

Reflow metals may cover some portions, or all, of the second leadframesurface. As an example, a tin alloy may cover at least the second endsof the lead segments, or alternatively all of the lead segments and theexposed outer chip pad surface.

In FIG. 3, dashed lines 330 indicate the locations, where a saw will cutthe completed leadframe strip into individual devices. The saw iscutting through encapsulation material 320 as well as through theleadframe segments.

Another embodiment of the invention is a method for fabricating aleadframe, which starts with the step of providing a base metalstructure with a plurality of surfaces and continues with the steps ofplating metal layers on these surfaces. The sequence of the consecutiveplating steps is:

Plating a layer of nickel on the base metal in the thickness range fromabout 0.5 to 2.0 μm.

Plating a layer of palladium on the nickel layer in the thickness rangefrom about 5 to 150 nm.

Plating a layer of tin on the palladium layer in the thickness rangefrom about 5 to 2 nm.

Some embodiments of the invention require selective plating. Examples ofthese embodiments are devices, which require an additional silver layerover the tin layer in selected areas; or devices, which require windowsin the tin layer so that the underlying palladium is exposed. Wheneverthe methods described above require a selective metal deposition of alayer onto the leadframe, an inexpensive, temporary masking step isused, which leaves only those leadframe portions exposed which areintended to receive the metal layer. Because of the fast plating time,conventional selective spot plating techniques can be considered,especially reusable rubber masks. For thin metal plating, a wheel systemis preferred.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. As an example, the invention applies to products using anytype of semiconductor chip, discrete or integrated circuit, and thematerial of the semiconductor chip may comprise silicon, silicongermanium, gallium arsenide, or any other semiconductor or compoundmaterial used in integrated circuit manufacturing.

As another example, the process step of stamping the leadframes from asheet of base metal may be followed by a process step of selectiveetching, especially of the exposed base metal surfaces in order tocreate large-area contoured surfaces for improved adhesion to moldingcompounds. The sequence of plated layers according to the invention canaccommodate any such specially etched leadframe base structures.

It is therefore intended that the appended claims encompass any suchmodifications or embodiment.

1. A leadframe strip comprising: a structure made of a base metal, saidstructure having a plurality of surfaces; and a stack of metal layersadherent to said base metal on each of said structure surfaces, saidstack comprising consecutively: a nickel layer covering substantiallysaid entire base metal surfaces; a palladium layer coveringsubstantially said entire nickel layer; and an outermost tin layercovering substantially said entire palladium layer.
 2. The leadframeaccording to claim 1 wherein said base metal is selected from a groupconsisting of copper, copper alloy, aluminum, iron-nickel alloy, andKovar.
 3. The leadframe according to claim 1 wherein said nickel layerhas a thickness between about 0.5 and 2.0 μm.
 4. The leadframe accordingto claim 1 wherein said palladium layer has a thickness between about 5and 150 nm.
 5. The leadframe according to claim 1 wherein said tin layerhas a thickness less than about 5 nm.
 6. The leadframe according toclaim 1 wherein said tin layer has a thickness of about 3 nm.
 7. Theleadframe according to claim 1 further having selective areas of silverlayer on said first surface suitable to metal welding and bonding. 8.The leadframe according to claim 1 wherein said outermost tin layer hasa plurality of windows in selected areas so that the underlyingpalladium is exposed for facilitating metal bond attachments.
 9. Asemiconductor device comprising: a leadframe having a structure made ofa base metal, said structure comprising a chip mount pad and a pluralityof lead segments; said base metal having consecutively a nickel layercovering substantially said base metal, a palladium coveringsubstantially said nickel layer, and an outermost tin layer coveringsubstantially said palladium layer, said base metal exposed at aplurality of outer sides of said device; a semiconductor chip attachedto said mount pad; conductive connections from said chip to said leadsegments; and polymeric encapsulation material covering said chip, saidconnections and portions of said lead segments.
 10. The device accordingto claim 9 further comprising reflow metals on those portions of saidsegments, which are not covered by said encapsulation material.
 11. Thedevice according to claim 9 wherein said connections are bonding wires.12. A method for fabricating a leadframe comprising the steps of:providing a structured base metal strip having a plurality of surfaces;and plating a stack of metal layers adherent to said base metal on eachof said structured surfaces, said step comprising consecutively: platinga nickel layer covering substantially said base metal; plating apalladium layer covering substantially said nickel layer; and plating anoutermost tin layer covering substantially said palladium layer.
 13. Theleadframe according to claim 12 wherein said nickel layer has athickness between about 0.5 and 2.0 μm.
 14. The method according toclaim 12 wherein said palladium layer has a thickness between about 5and 150 nm.
 15. The method according to claim 12 wherein said tin layerhas a thickness less than about 5 nm.
 16. The method according to claim12 wherein said tin layer has a thickness of about 3 nm.
 17. The methodaccording to claim 12 further comprising the step of plating said tinlayer so that it leaves selected windows open to the underlyingpalladium layer.
 18. The method according to claim 12 further comprisingthe step of selectively plating a layer of silver in contact with saidtin layer.